This nonprovisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No. 90128662 filed in TAIWAN on Nov. 20, 2001, which is herein incorporated by reference.
1. Field of the Invention
The invention relates to a stress wave testing method for wood, and more particularly to a non-destructive stress wave testing method for wood.
2. Description of the Related Art
Generally speaking, the moisture content, density and modulus of elasticity of wood can be measured by destructive and non-destructive testing methods. The non-destructive testing method means that the properties or inner structure of wood are measured without destroying the wood. For example, a positive linear relationship exists between the strength and the modulus of elasticity of wood. The strength of wood can be determined by the modulus of elasticity thereof. The techniques for measuring the modulus of elasticity include vibration, ultrasonic and stress wave methods. Further, a tap tone method is also provided to measure resonance frequency of wood according to the spectrum measured by an FFT spectrum analyzer. Then, the speed of sound in the wood and the modulus of elasticity of the wood can be determined according to the resonance frequency.
When one end of a timber is struck, stress waves generated therein are measured by a conventional non-destructive testing method. The conventional non-destructive testing method uses an acceleration transducer to measure the time difference of acceleration waveform between the two ends. Then, the speed of sound in the timber and the modulus of elasticity of the timber are determined according to the time difference. Nevertheless, the acceleration transducer is very expensive and easily damaged, thus reducing reliability thereof.
The present invention is directed to a non-destructive stress wave testing method for wood.
Accordingly, the present invention provides a non-destructive stress wave testing method for wood. A timber having a first end and a second end is provided. A first strain gauge and a second strain gauge are disposed on the first end and the second end, respectively. The first end of the timber is struck to generate an impact compression stress wave. The impact compression stress wave moves toward the second end of the timber along the longitudinal direction thereof and transforms into a tension stress wave on the second end to return to the first end, such that the impact compression stress wave moves between the first end and the second end. The resonance frequency of the timber is sensed by the first strain gauge and the second strain gauge. The time difference of the impact compression stress wave passing through the first strain gauge and the second strain gauge is measured by an oscilloscope. The resonance frequency of the timber is displayed on an FFT spectrum analyzer. Then, the speed of sound in the timber is determined according to the distance between the first and second strain gauges and the time difference, or according to the length and the resonance frequency thereof. The modulus of elasticity of the timber is determined according to the speed of sound therein and the density thereof.